/******************************************************************************
 * Spine Runtimes License Agreement
 * Last updated January 1, 2020. Replaces all prior versions.
 *
 * Copyright (c) 2013-2020, Esoteric Software LLC
 *
 * Integration of the Spine Runtimes into software or otherwise creating
 * derivative works of the Spine Runtimes is permitted under the terms and
 * conditions of Section 2 of the Spine Editor License Agreement:
 * http://esotericsoftware.com/spine-editor-license
 *
 * Otherwise, it is permitted to integrate the Spine Runtimes into software
 * or otherwise create derivative works of the Spine Runtimes (collectively,
 * "Products"), provided that each user of the Products must obtain their own
 * Spine Editor license and redistribution of the Products in any form must
 * include this license and copyright notice.
 *
 * THE SPINE RUNTIMES ARE PROVIDED BY ESOTERIC SOFTWARE LLC "AS IS" AND ANY
 * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
 * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
 * DISCLAIMED. IN NO EVENT SHALL ESOTERIC SOFTWARE LLC BE LIABLE FOR ANY
 * DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
 * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES,
 * BUSINESS INTERRUPTION, OR LOSS OF USE, DATA, OR PROFITS) HOWEVER CAUSED AND
 * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
 * THE SPINE RUNTIMES, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 *****************************************************************************/

using System;

namespace Spine
{
    public class Triangulator
    {
        private readonly ExposedList<ExposedList<float>> convexPolygons = new ExposedList<ExposedList<float>>();
        private readonly ExposedList<ExposedList<int>> convexPolygonsIndices = new ExposedList<ExposedList<int>>();

        private readonly ExposedList<int> indicesArray = new ExposedList<int>();
        private readonly ExposedList<bool> isConcaveArray = new ExposedList<bool>();
        private readonly ExposedList<int> triangles = new ExposedList<int>();

        private readonly Pool<ExposedList<float>> polygonPool = new Pool<ExposedList<float>>();
        private readonly Pool<ExposedList<int>> polygonIndicesPool = new Pool<ExposedList<int>>();

        public ExposedList<int> Triangulate(ExposedList<float> verticesArray)
        {
            var vertices = verticesArray.Items;
            int vertexCount = verticesArray.Count >> 1;

            var indicesArray = this.indicesArray;
            indicesArray.Clear();
            int[] indices = indicesArray.Resize(vertexCount).Items;
            for (int i = 0; i < vertexCount; i++)
                indices[i] = i;

            var isConcaveArray = this.isConcaveArray;
            bool[] isConcave = isConcaveArray.Resize(vertexCount).Items;
            for (int i = 0, n = vertexCount; i < n; ++i)
                isConcave[i] = IsConcave(i, vertexCount, vertices, indices);

            var triangles = this.triangles;
            triangles.Clear();
            triangles.EnsureCapacity(Math.Max(0, vertexCount - 2) << 2);

            while (vertexCount > 3)
            {
                // Find ear tip.
                int previous = vertexCount - 1, i = 0, next = 1;

                // outer:
                while (true)
                {
                    if (!isConcave[i])
                    {
                        int p1 = indices[previous] << 1, p2 = indices[i] << 1, p3 = indices[next] << 1;
                        float p1x = vertices[p1], p1y = vertices[p1 + 1];
                        float p2x = vertices[p2], p2y = vertices[p2 + 1];
                        float p3x = vertices[p3], p3y = vertices[p3 + 1];
                        for (int ii = (next + 1) % vertexCount; ii != previous; ii = (ii + 1) % vertexCount)
                        {
                            if (!isConcave[ii]) continue;
                            int v = indices[ii] << 1;
                            float vx = vertices[v], vy = vertices[v + 1];
                            if (PositiveArea(p3x, p3y, p1x, p1y, vx, vy))
                            {
                                if (PositiveArea(p1x, p1y, p2x, p2y, vx, vy))
                                {
                                    if (PositiveArea(p2x, p2y, p3x, p3y, vx, vy)) goto break_outer; // break outer;
                                }
                            }
                        }
                        break;
                    }
                break_outer:

                    if (next == 0)
                    {
                        do
                        {
                            if (!isConcave[i]) break;
                            i--;
                        } while (i > 0);
                        break;
                    }

                    previous = i;
                    i = next;
                    next = (next + 1) % vertexCount;
                }

                // Cut ear tip.
                triangles.Add(indices[(vertexCount + i - 1) % vertexCount]);
                triangles.Add(indices[i]);
                triangles.Add(indices[(i + 1) % vertexCount]);
                indicesArray.RemoveAt(i);
                isConcaveArray.RemoveAt(i);
                vertexCount--;

                int previousIndex = (vertexCount + i - 1) % vertexCount;
                int nextIndex = i == vertexCount ? 0 : i;
                isConcave[previousIndex] = IsConcave(previousIndex, vertexCount, vertices, indices);
                isConcave[nextIndex] = IsConcave(nextIndex, vertexCount, vertices, indices);
            }

            if (vertexCount == 3)
            {
                triangles.Add(indices[2]);
                triangles.Add(indices[0]);
                triangles.Add(indices[1]);
            }

            return triangles;
        }

        public ExposedList<ExposedList<float>> Decompose(ExposedList<float> verticesArray, ExposedList<int> triangles)
        {
            var vertices = verticesArray.Items;
            var convexPolygons = this.convexPolygons;
            for (int i = 0, n = convexPolygons.Count; i < n; i++)
            {
                polygonPool.Free(convexPolygons.Items[i]);
            }
            convexPolygons.Clear();

            var convexPolygonsIndices = this.convexPolygonsIndices;
            for (int i = 0, n = convexPolygonsIndices.Count; i < n; i++)
            {
                polygonIndicesPool.Free(convexPolygonsIndices.Items[i]);
            }
            convexPolygonsIndices.Clear();

            var polygonIndices = polygonIndicesPool.Obtain();
            polygonIndices.Clear();

            var polygon = polygonPool.Obtain();
            polygon.Clear();

            // Merge subsequent triangles if they form a triangle fan.
            int fanBaseIndex = -1, lastWinding = 0;
            int[] trianglesItems = triangles.Items;
            for (int i = 0, n = triangles.Count; i < n; i += 3)
            {
                int t1 = trianglesItems[i] << 1, t2 = trianglesItems[i + 1] << 1, t3 = trianglesItems[i + 2] << 1;
                float x1 = vertices[t1], y1 = vertices[t1 + 1];
                float x2 = vertices[t2], y2 = vertices[t2 + 1];
                float x3 = vertices[t3], y3 = vertices[t3 + 1];

                // If the base of the last triangle is the same as this triangle, check if they form a convex polygon (triangle fan).
                var merged = false;
                if (fanBaseIndex == t1)
                {
                    int o = polygon.Count - 4;
                    float[] p = polygon.Items;
                    int winding1 = Winding(p[o], p[o + 1], p[o + 2], p[o + 3], x3, y3);
                    int winding2 = Winding(x3, y3, p[0], p[1], p[2], p[3]);
                    if (winding1 == lastWinding && winding2 == lastWinding)
                    {
                        polygon.Add(x3);
                        polygon.Add(y3);
                        polygonIndices.Add(t3);
                        merged = true;
                    }
                }

                // Otherwise make this triangle the new base.
                if (!merged)
                {
                    if (polygon.Count > 0)
                    {
                        convexPolygons.Add(polygon);
                        convexPolygonsIndices.Add(polygonIndices);
                    }
                    else
                    {
                        polygonPool.Free(polygon);
                        polygonIndicesPool.Free(polygonIndices);
                    }
                    polygon = polygonPool.Obtain();
                    polygon.Clear();
                    polygon.Add(x1);
                    polygon.Add(y1);
                    polygon.Add(x2);
                    polygon.Add(y2);
                    polygon.Add(x3);
                    polygon.Add(y3);
                    polygonIndices = polygonIndicesPool.Obtain();
                    polygonIndices.Clear();
                    polygonIndices.Add(t1);
                    polygonIndices.Add(t2);
                    polygonIndices.Add(t3);
                    lastWinding = Winding(x1, y1, x2, y2, x3, y3);
                    fanBaseIndex = t1;
                }
            }

            if (polygon.Count > 0)
            {
                convexPolygons.Add(polygon);
                convexPolygonsIndices.Add(polygonIndices);
            }

            // Go through the list of polygons and try to merge the remaining triangles with the found triangle fans.
            for (int i = 0, n = convexPolygons.Count; i < n; i++)
            {
                polygonIndices = convexPolygonsIndices.Items[i];
                if (polygonIndices.Count == 0) continue;
                int firstIndex = polygonIndices.Items[0];
                int lastIndex = polygonIndices.Items[polygonIndices.Count - 1];

                polygon = convexPolygons.Items[i];
                int o = polygon.Count - 4;
                float[] p = polygon.Items;
                float prevPrevX = p[o], prevPrevY = p[o + 1];
                float prevX = p[o + 2], prevY = p[o + 3];
                float firstX = p[0], firstY = p[1];
                float secondX = p[2], secondY = p[3];
                int winding = Winding(prevPrevX, prevPrevY, prevX, prevY, firstX, firstY);

                for (int ii = 0; ii < n; ii++)
                {
                    if (ii == i) continue;
                    var otherIndices = convexPolygonsIndices.Items[ii];
                    if (otherIndices.Count != 3) continue;
                    int otherFirstIndex = otherIndices.Items[0];
                    int otherSecondIndex = otherIndices.Items[1];
                    int otherLastIndex = otherIndices.Items[2];

                    var otherPoly = convexPolygons.Items[ii];
                    float x3 = otherPoly.Items[otherPoly.Count - 2], y3 = otherPoly.Items[otherPoly.Count - 1];

                    if (otherFirstIndex != firstIndex || otherSecondIndex != lastIndex) continue;
                    int winding1 = Winding(prevPrevX, prevPrevY, prevX, prevY, x3, y3);
                    int winding2 = Winding(x3, y3, firstX, firstY, secondX, secondY);
                    if (winding1 == winding && winding2 == winding)
                    {
                        otherPoly.Clear();
                        otherIndices.Clear();
                        polygon.Add(x3);
                        polygon.Add(y3);
                        polygonIndices.Add(otherLastIndex);
                        prevPrevX = prevX;
                        prevPrevY = prevY;
                        prevX = x3;
                        prevY = y3;
                        ii = 0;
                    }
                }
            }

            // Remove empty polygons that resulted from the merge step above.
            for (int i = convexPolygons.Count - 1; i >= 0; i--)
            {
                polygon = convexPolygons.Items[i];
                if (polygon.Count == 0)
                {
                    convexPolygons.RemoveAt(i);
                    polygonPool.Free(polygon);
                    polygonIndices = convexPolygonsIndices.Items[i];
                    convexPolygonsIndices.RemoveAt(i);
                    polygonIndicesPool.Free(polygonIndices);
                }
            }

            return convexPolygons;
        }

        static private bool IsConcave(int index, int vertexCount, float[] vertices, int[] indices)
        {
            int previous = indices[(vertexCount + index - 1) % vertexCount] << 1;
            int current = indices[index] << 1;
            int next = indices[(index + 1) % vertexCount] << 1;
            return !PositiveArea(vertices[previous], vertices[previous + 1], vertices[current], vertices[current + 1], vertices[next],
                vertices[next + 1]);
        }

        static private bool PositiveArea(float p1x, float p1y, float p2x, float p2y, float p3x, float p3y)
        {
            return p1x * (p3y - p2y) + p2x * (p1y - p3y) + p3x * (p2y - p1y) >= 0;
        }

        static private int Winding(float p1x, float p1y, float p2x, float p2y, float p3x, float p3y)
        {
            float px = p2x - p1x, py = p2y - p1y;
            return p3x * py - p3y * px + px * p1y - p1x * py >= 0 ? 1 : -1;
        }
    }
}
